Recently, there have been many products on the market which use solid state image sensors at the image-forming plane of various sorts of video cameras and electronic still cameras. Due to technological advancements, there has been a trend toward greater miniaturization and, accompanying this trend, smaller image-forming lenses with wider imaging angles have been required. Also, it is strongly desired that the smaller image-forming lenses provide a bright image (i.e., that the lenses have a low F-number, F.sub.NO), and be low in cost. As such an image-forming lens, an image-forming lens having only two lens elements has previously been disclosed by this inventor (Japanese Laid Open Patent Application H09-25900). In this inventor's prior image-forming lens, the object-side surface of the first lens element in order from the object side (hereinafter termed the "first surface") is made to be concave.
On the other hand, because the trend in solid state image sensors has been toward higher image resolution with more pixels, imaging lenses having higher resolution are being demanded. Furthermore, because these cameras are now often used for recording written documents having lines of text with parallel borders in which lens distortion is particularly noticeable, it has become important to suppress distortion below an extremely low level while sultaneously providing a wide image angle. However, the above-mentioned lens having only two lens elements does not have a sufficiently small distortion to satisfy these demands.
Although the above-described patent application by the inventor of the present application employs a lens of only two lens elements in which the first surface is concave, the present invention employs a convex first surface. Four patent applications known to applicant, namely Japanese Laid Open Patent Applications H10-90597, H7-50246 and H6-67089, and U.S. Pat. No. 5,812,327, each disclose a lens having only two lens elements wherein a convex surface is employed for the first surface. However, the lens described in Japanese Laid Open Patent Application H7-50246 and the lens described in U.S. Pat. No. 5,812,327 each has large distortion. On the other hand, the lens described in Japanese Laid Open Patent Application H6-67089 has small distortion but a narrow image angle. Similarly, the lens described in Japanese Laid Open Patent Application H10-90597 has an image angle of only 50 degrees, as compared to the lens of the present invention which has an image angle above 60 degrees. Therefore, neither of these lenses is suitable for use with state-of-the-art image sensors in applications having an image angle of more than 50 degrees.